DE 2 815 684 [GB 1,557,854] describes a tap changer for transformers that comprises a tap selector switch with a plurality of fixed contacts for connection with a tapped regulating winding of the transformer, a movable contact for successive selection of the fixed contacts and a changeover switch for switching over the winding connections when a predetermined fixed contact has been selected. This tap changer comprises an interruption device that is actuatable when the other contacts are selected so as to produce a relative movement between the engaged contacts of the changeover switch without switching over the changeover switch.
DE 2 815 684 also explains the following: The transformer can operate over lengthy periods without a larger regulating range being required, for which reason the changeover switch is consequently actuated only rarely. During these lengthy periods the changeover switch conducts current and oxidizing occurs, as a result of which the contact resistance increases. In such cases, the switch can be destroyed by the resulting heat. During each change of the tap, a pinion rotates through 90°. An arm mounted eccentrically thereto allows the changeover switch a slight movement. This movement is not sufficient to actuate the changeover switch, i.e. switch over the contacts, but it is sufficient to clean the contacts. The said changeover switch can be any kind of current-conducting switch, from which it is assumed that it is not frequently actuated in the installation. The switch is usually a reversing switch for reversing the directional sense of the regulating winding relative to the main winding or a coarse/fine switch for switching an additional winding between the regulating winding and the main winding. The contacts of the changeover switch are disturbed or influenced by the described device during each tap change, this being advantageous, but not essential.
However, this permanent relative movement leads to significant shortening of the service life of the changeover switch and, in particular, its contacts.